Elevators are presently provided with a plurality of braking devices which are designed for use in normal operation of an elevator, for example to hold the elevator car in place where it stops at a landing, and which are designed for use in emergency situations such as arresting the motion of a free-falling elevator car.
One such braking device is provided to slow an overspeeding elevator car, that is one which is travelling over a predetermined rate. Such braking devices typically employ a governor device which triggers the operation of safeties. In such elevator systems a governor rope is provided which is looped over a governor sheave at the top of the hoistway and a tension sheave is at the bottom of the hoistway and is also attached to the elevator car. When the governor rope exceeds the predetermined rate of the elevator car the governor grabs the governor rope, pulling two rods connected to the car. The rods pull two wedge shaped safeties which pinch the guide rail on which the elevator car rides thereby braking and slowing the elevator car.
The device and method employed in determining an overspeed condition of an elevator car is important to the proper working of the safety braking system. In conventional systems the speed of an elevator car may be monitored through the governor rope, governor sheave, tension sheave or mechanical linkages which operate the safeties. For instance, the governor sheave described above typically employs a centrifugal device which when an overspeed condition is reached engages a brake producing drag on the governor rope and thereby activating the safeties. The governor rope rotates a governor, at a rate of rotary speed that relates to the linear speed of the elevator car. The governor has fly weights that move outwardly with increasing speed as a result of increasing centrifugal force. When the elevator exceeds a predetermined speed the fly weights trip an overspeed switch which allows a set ofjaws to grip the rope and activate the safeties. In other systems a tachometer is attached to a secondary cable attached to the sheave and employed to monitor an overspeed condition of the elevator car and activate the safeties.
A disadvantage of the prior art systems is the wear which occurs to the rope and governor systems. The greatest problem with this type of wear is that it is often visually undetectable. In addition when an overspeed condition occurs the elevator is required to be taken out of service until a mechanic is available to reset the governor unit and release the safeties.
Another disadvantage of a governor rope assembly is the required maintenance and hoistway space required. The governor rope, sheaves and linkages must be periodically cleaned, lubricated and replaced. All maintenance requirements are considered burdensome to those skilled in the art, and therefore an undesirable feature. As such there is a need to eliminate a governor rope assembly and a further need for an accurate device and method to monitor and determine an overspeed condition of an elevator car without a governor rope assembly. In light of this need there exists a further need for an accurate, non-contact, continuous and instantaneous device and method of detecting an overspeed condition of an elevator.